# The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII

## TL;DR

This study investigates how removing specific proteins affects the rate and pattern of oxygen release in photosystem II of tobacco plants.

## Contribution

A novel measurement protocol was developed to directly observe the kinetics of oxygen release from PSII.

## Key findings

- Oxygen evolution in PSII occurs in two distinct phases: fast (~ms) and slow (~tens of ms).
- Removing external proteins PsbO, PsbP, and PsbQ slows the fast phase and accelerates the slow phase of oxygen release.
- The biphasic nature of oxygen evolution is independent of the OEC state distribution model.

## Abstract

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.